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CM4212 – TM OM
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Carbonyls
1. Homoleptic carbonyl complexes exhibit only one CO band.
False
True
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The number of carbonyl bands depends on the number of unique carbonyl ligands, which in turn is influenced by geometry and symmetry.
2. Carbonyl complexes are favorably formed with low valent metal centers.
True
False
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Carbonyl ligands are good pi-acceptors. Low valent metal centers provide sufficient electron density for favorable pi-backbonding to bound carbonyl ligands.
3. Nucleophilic attack of a bound carbonyl occurs at...
the oxygen atom.
the carbon atom.
the CO triple bond.
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Upon donation of the lone pair to the metal, there will be partial positive charge at the carbon atom, which makes it prone to attack by nucleophiles.
4. The number of carbonyl bands in the IR spectrum is indicative of the number of CO ligands in the complex.
True
False
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This is not the case. A octahedral, homoleptic hexacarbonyl complex of the type [M(CO)6] exhibits only one band.
5. Homoleptic carbonyl complexes exhibit only one CO band.
True
False
Show answer
The number of bands depends on the structure of the complex and the number of unique CO ligands. A trigonal bipyramidal complex [M(CO)5] would have two types of unique COs and more than one band will be observed.
6. What does the Tolman Electronic Parameter measure?
It measures the amount of backdonation to phosphines in [Ni(CO)3L] complexes.
It measures the amount of sigma donation of phosphines in [Ni(CO)3L] complexes.
It measures the amount of backdonation to CO in [Ni(CO)3L] complexes.
It measures the amount of sigma donation of CO ligands in [Ni(CO)3L] complexes.
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The TEP is the A1 carbonyl stretch in complexes of the type [Ni(CO)3L]. It is therefore directly linked to the amount of backdonation to the carbonyl ligands.
7. Reduction of a metal-carbonyl complex generally leads to...
increase of M–C distance.
decrease of M–C distance.
increase of C–O distance.
decrease of C–O distance.
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Backdonation from metal to carbonyl strengthens the metal-carbon bond, but weakens the carbon-oxygen bond.
8. Coordination of a carbonyl ligand to a low valent metal generally leads to...
shortening of the C-O bond.
lengthening of the C-O bond.
no change of the C-O bond.
Show answer
Carbonyl is a very good pi-acceptor ligand. Coordination to a low valent (i.e. electron-rich) metal center will lead to backdonation into the antibonding pi* orbital, thus weakening the carbon-oxygen bond.
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